Hydra

A giant monster with seven, nine, or more heads — the offspring of Typhon and Echidna, and which was brought up by Hera. It ravaged the country of Lernae near Argos, and dwelt in a swamp near the well of Amymone.

Heracles was sent to kill the monster as second of his Twelve Labors (see Lernaean Hydra), but each time he cut off one head, two grew back in its place. Eventually he burned away the heads, and buried the ninth or immortal one under a huge rock.